What Are GIP and GLP-1 and How Do They Work?

GIP and GLP-1 are two hormones your gut releases after you eat, and their main job is to amplify insulin secretion so your body can handle the surge of glucose from a meal. Together they account for what scientists call the “incretin effect,” a phenomenon noticed even before insulin was discovered: glucose taken by mouth triggers far more insulin than the same amount of glucose delivered directly into a vein.¹ These two peptides have become the foundation of some of the most talked-about medications in modern medicine, but understanding what they actually do in your body, and where they diverge, makes the pharmacology far more intuitive.

Where They Come From

GLP-1, or glucagon-like peptide-1, is a 30-amino-acid hormone made by specialized L-cells scattered along the intestinal lining. It is produced when those cells process a larger precursor protein called proglucagon, and meals are the primary trigger for its release.² GIP, or glucose-dependent insulinotropic polypeptide, is a slightly longer peptide at 42 amino acids, secreted by K-cells concentrated in the upper part of the small intestine.³ Both hormones spike within minutes of eating, but GIP levels tend to climb faster and higher. In healthy people, fasting GIP sits around 5 to 20 picomoles per liter, then jumps to roughly 50 to 100 picomoles within half an hour of drinking a standard glucose solution. A mixed meal pushes GIP even higher, reaching 100 to 150 picomoles within an hour.³ Protein-rich foods stimulate GIP release more rapidly than fat-rich foods of the same calorie content.

Neither hormone lasts long in the bloodstream. An enzyme called DPP-4 chops both of them into inactive fragments within a few minutes, which is why early drug development focused on either blocking DPP-4 or engineering synthetic versions of GLP-1 that resist breakdown.

What They Do in the Pancreas

The headline act for both hormones is boosting insulin. When GIP or GLP-1 binds to its receptor on a pancreatic beta cell, it raises levels of a signaling molecule called cyclic AMP inside the cell, which primes the cell to release more insulin in response to glucose.⁴ The crucial detail is that this only happens when blood sugar is elevated. If glucose is low, neither hormone forces insulin out. That glucose-dependent safety switch is a big part of why incretin-based drugs carry a lower risk of dangerous blood sugar crashes compared to older diabetes medications.

The two hormones part ways when it comes to glucagon, the counter-regulatory hormone that tells the liver to release stored sugar. GLP-1 suppresses glucagon when blood sugar is above normal fasting levels, which helps keep post-meal glucose from spiking too high. GIP does the opposite: it promotes glucagon secretion, especially when blood sugar is low.⁵ ⁶ This sounds contradictory if you think of GIP as just an insulin booster, but it actually makes biological sense: GIP helps fine-tune blood sugar in both directions. After a meal, when glucose is high, GIP’s insulin-boosting effect dominates. Between meals, when glucose starts dipping, GIP’s glucagon-promoting side helps prevent hypoglycemia.

Mouse experiments illustrate how essential both hormones are. In female mice engineered to lack both the GIP and GLP-1 receptors, the incretin effect vanished entirely: the extra insulin release normally triggered by oral glucose was completely absent.⁷

Beyond the Pancreas

If GIP and GLP-1 only affected insulin, they would still be interesting, but the reason they have become central to obesity treatment is that their influence extends well beyond the pancreas.

GLP-1 acts on the brain. Receptors for GLP-1 are found in brain regions that regulate appetite, and research has shown that weight loss driven by GLP-1 receptor agonists depends on those central nervous system receptors.⁸ More recent work has identified the dorsomedial hypothalamus as a key target, where GLP-1 receptor activation appears to promote a feeling of fullness even before food is consumed, what researchers call “pre-ingestive cognitive satiation.”⁹ In practical terms, people on GLP-1 medications often describe food simply being less interesting, a shift that feels cognitive as much as physical.

GLP-1 also slows gastric emptying, the rate at which your stomach pushes food into the small intestine. A meta-analysis of studies using imaging found that people on GLP-1 receptor agonists had a gastric half-emptying time roughly 36 minutes longer than those on placebo.¹⁰ Slower gastric emptying has a dual effect: it blunts post-meal blood sugar spikes and contributes to the feeling of fullness that lingers after eating. The downside is that it can cause nausea, and it has implications for anyone undergoing procedures that require an empty stomach.¹¹

GIP, meanwhile, has a distinct territory that GLP-1 does not share: fat tissue. The GIP receptor is expressed in adipose tissue, while the GLP-1 receptor is not.¹² For a long time, this made GIP seem counterproductive for weight loss, because early thinking was that GIP signaling in fat cells just helped store more fat. That picture has changed dramatically. A 2024 study created mice whose fat cells could be induced to express the GIP receptor at high levels. Activating those receptors in obese mice triggered about 35% weight loss, along with increased fat burning, thermogenesis, and energy expenditure.¹³ The mechanism involved a process where fat cells burn energy by cycling calcium back and forth across internal membranes without producing useful work, essentially converting stored energy into heat.

How Drugs Exploit Both Receptors

The first wave of incretin-based medications targeted only GLP-1. Drugs like exenatide, liraglutide, and semaglutide are synthetic GLP-1 receptor agonists, engineered to resist the rapid breakdown that limits the natural hormone’s lifespan. They deliver the appetite-suppressing, insulin-boosting, and gastric-slowing effects of GLP-1 at much higher and more sustained levels than the body produces on its own.

The newer frontier is dual agonism, hitting both the GIP and GLP-1 receptors simultaneously. Tirzepatide, the first approved dual agonist, is not a 50-50 blend of the two signals. Pharmacological analysis shows it favors the GIP receptor more heavily than the GLP-1 receptor. At the GLP-1 receptor, tirzepatide also behaves differently from natural GLP-1: it is biased toward generating the cyclic AMP signal that drives insulin secretion while triggering less of the beta-arrestin pathway that normally dampens the receptor’s response.¹⁴ In plainer terms, tirzepatide gets more insulin out of the GLP-1 receptor per dose because it sidesteps one of the receptor’s built-in braking mechanisms. That imbalanced design, leaning toward GIP with a tweaked GLP-1 signal, may help explain why tirzepatide has shown larger effects on weight loss and blood sugar control than GLP-1-only drugs in clinical trials.

An even newer molecule, retatrutide, adds a third receptor: the glucagon receptor. In a phase 2 trial, the highest dose of this triple agonist produced roughly 24% weight loss over 48 weeks in people with obesity.¹⁵ Glucagon receptor activation boosts energy expenditure and fat burning in the liver, so stacking it on top of GIP and GLP-1 signaling adds a distinct metabolic lever. Phase 3 trials are still ongoing.

Cardiovascular and Kidney Effects

GLP-1 receptors are not confined to the gut, brain, and pancreas. They also sit on blood vessel walls, and activating them appears to reduce vascular inflammation. In mice with high blood pressure, the GLP-1 agonist liraglutide normalized blood pressure, reduced cardiac enlargement, and improved the function of blood vessel linings. The protective effect depended specifically on GLP-1 receptors in the endothelium, the inner lining of blood vessels, rather than in immune cells.¹⁶ Large cardiovascular outcome trials in humans have since confirmed that certain GLP-1 agonists reduce the risk of major cardiovascular events in people with type 2 diabetes.

The kidneys are another area of growing interest. GLP-1 receptor expression in the kidney is modest, but the downstream effects of GLP-1 agonists on blood pressure, inflammation, and blood sugar all converge in ways that seem to protect kidney function. Researchers are actively studying whether GLP-1 agonists can slow the progression of chronic kidney disease in people with diabetes and obesity.¹⁷

Side Effects and the Gut

The most common side effects of GLP-1 receptor agonists are gastrointestinal: nausea, vomiting, diarrhea, and constipation.¹⁸ These tend to be dose-dependent, meaning they are worse at higher doses and during the first weeks of treatment, and they often improve as the body adjusts.¹⁹ The nausea, in particular, is closely tied to the slowed gastric emptying that is part of how these drugs work. Food sitting in the stomach longer than usual simply feels uncomfortable at first.

More serious but rarer concerns include gastroparesis-like symptoms, where the stomach empties extremely slowly, and case reports of acute kidney injury linked primarily to severe dehydration from persistent vomiting.¹⁸ Biliary disease and pancreatic safety have been flagged in some studies, though the evidence across trials is inconsistent and sometimes contradictory.¹⁹ For people on insulin or sulfonylureas, the blood-sugar-lowering effects of GLP-1 therapy combined with slowed gastric emptying can increase the risk of hypoglycemia, which is why dose adjustments to existing medications are often necessary.¹¹

What Happens When You Stop

One of the most common questions about incretin-based drugs is whether the weight comes back after stopping. The short answer is that it frequently does, and the biology helps explain why. When pharmacological GLP-1 signaling is withdrawn, appetite tends to return, partly because the brain’s appetite-regulating circuits lose the suppressive signal they had been receiving. At the same time, hunger-promoting hormones like ghrelin may rebound, creating a window where the biological drive to eat outstrips any learned habits of eating less.²⁰

Other proposed mechanisms include metabolic adaptation, where the body’s energy expenditure decreases after weight loss, and possible changes in how the pancreas handles insulin after long-term exposure to GLP-1 agonists.²¹ The upshot is that abrupt discontinuation tends to produce faster weight regain than gradual tapering, and many clinicians now treat obesity with these drugs much the way they treat blood pressure with antihypertensives: as a long-term or lifelong intervention rather than a short course.²²

Muscle Mass and Body Composition

Whenever significant weight loss occurs, whether from surgery, calorie restriction, or medication, some of that weight is lean tissue rather than fat. This has been a persistent concern with GLP-1 agonists. However, recent data paints a more reassuring picture. Studies using MRI suggest that the muscle loss seen with GLP-1 receptor agonists is roughly proportional to what you would expect for the amount of weight lost, the person’s age, and their health status, rather than representing excessive muscle wasting.²³ Improvements in insulin sensitivity and reductions in fat infiltration within muscles may actually improve muscle quality even as total body weight drops.

For tirzepatide specifically, a systematic review found that the drug preferentially reduces fat mass while relatively preserving lean mass, and markers of muscle composition stayed stable or improved.²⁴ Whether this advantage comes from the GIP component, from the drug’s unique receptor pharmacology, or simply from the pattern of weight loss it produces is still being studied. Incretin-based therapy in general appears to preferentially strip fat while sparing lean tissue, with potential improvements in muscle quality through reduced fat buildup within the muscle itself.²⁵ Resistance exercise during treatment remains the strongest known strategy for preserving muscle, regardless of which medication is being used.

Adolescents and Growing Bodies

The use of these drugs in teenagers has produced striking results but also unique questions. In the STEP TEENS trial, adolescents aged 12 to 18 treated with once-weekly semaglutide lost an average of about 16% of their body weight over 68 weeks, compared with a slight weight gain in the placebo group. Nearly three-quarters of treated teens achieved at least 10% weight reduction.²⁶

The concern specific to adolescents is that puberty involves rapid changes in body composition, bone mineralization, and muscle accrual, all influenced by hormones like insulin, growth hormone, and sex steroids. Suppressing appetite and altering energy balance during this developmental window could, in theory, interfere with those processes. Current trial data does not provide enough detail on lean mass trajectories or pubertal staging to settle the question.²⁷ Future studies will need to track skeletal growth and body composition changes over longer periods before clinicians can be fully confident that effective weight loss in teenagers does not come at the expense of healthy development.

Neuroprotection Research

One of the more surprising branches of incretin research has nothing to do with metabolism at all. Both GLP-1 and GIP receptors are found in the brain, and animal studies have consistently shown that drugs targeting these receptors reduce inflammation, oxidative stress, and nerve cell death in models of Alzheimer’s and Parkinson’s disease.²⁸ Dual GLP-1/GIP agonists appear to outperform single GLP-1 drugs in these models, protecting memory formation, synaptic connections, and dopamine-producing neurons. One dual agonist tested in a Parkinson’s mouse model reversed motor impairments and boosted levels of BDNF, a protein critical for nerve cell survival and growth.²⁹ A triple agonist targeting GLP-1, GIP, and glucagon receptors has also reduced amyloid-beta accumulation and brain inflammation in an Alzheimer’s mouse model.³⁰

These are still animal findings, and the leap from mice to human neurodegenerative disease is long and full of failures. But the consistency of the results across different labs and disease models, combined with the fact that several of these drugs are already approved for human use in other contexts, has made this one of the more closely watched areas in neurology. Human trials exploring whether GLP-1 agonists slow cognitive decline are underway, and their outcomes will determine whether the neuroprotective promise translates beyond the lab.

An Evolutionary Footnote

The GIP and GLP-1 systems are remarkably old. The genes encoding GIP and its receptor appear in species across most vertebrate classes, suggesting the system was in place early in vertebrate evolution. Interestingly, two groups, cartilaginous fish like sharks and birds, retained the gene for GIP itself but lost the gene for the GIP receptor, meaning they produce the hormone without the matching lock for it to activate.³¹ Why a hormone would be conserved after its receptor disappears remains an open question. It might serve functions through related receptors, or it may simply be a genetic fossil that has not yet been fully purged. Either way, the deep evolutionary roots of these hormones hint at how fundamental nutrient-sensing and insulin regulation are to vertebrate life, far predating the metabolic diseases these hormones are now being marshaled to treat.

1
PubMed Central. The Role of Incretins on Insulin Function and Glucose Homeostasis
2
PubMed. The physiology of glucagon-like peptide 1
3
PubMed Central. GIP and GLP‐1, the two incretin hormones: Similarities and differences
4
PubMed. GLP-1 and GIP receptor signaling in beta cells – A review of receptor interactions and co-stimulation
5
PubMed. The evolving story of incretins (GIP and GLP-1) in metabolic and cardiovascular disease: A pathophysiological update
6
PubMed Central. The Effects of Dual GLP-1/GIP Receptor Agonism on Glucagon Secretion-A Review
7
PubMed Central. The Incretin Effect in Female Mice With Double Deletion of GLP-1 and GIP Receptors
8
PubMed Central. Glucagon-like peptide-1 receptors in the brain: controlling food intake and body weight
9
PubMed Central. Glucagon-Like Peptide-1 and Hypothalamic Regulation of Satiation: Cognitive and Neural Insights from Human and Animal Studies
10
PubMed Central. Quantified Metrics of Gastric Emptying Delay by Glucagon-Like Peptide-1 Agonists: A Systematic Review and Meta-Analysis With Insights for Periprocedural Management
11
PubMed Central. Physiology and Pharmacology of Effects of GLP-1-based Therapies on Gastric, Biliary and Intestinal Motility
12
PubMed Central. The interplay of glucose-dependent insulinotropic polypeptide in adipose tissue
13
PubMed Central. The GIP receptor activates futile calcium cycling in white adipose tissue to increase energy expenditure and drive weight loss in mice
14
The Journal of Clinical Investigation. Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist
15
PubMed Central. The Road towards Triple Agonists: Glucagon-Like Peptide 1, Glucose-Dependent Insulinotropic Polypeptide and Glucagon Receptor – An Update
16
PubMed Central. Endothelial GLP-1 Receptor Mediates Cardiovascular Protection by Liraglutide In Mice With Experimental Arterial Hypertension
17
PubMed Central. GLP-1 agonists in the treatment of chronic kidney disease in type 2 diabetes and obesity
18
PubMed Central. Adverse Effects of GLP-1 Receptor Agonists
19
PubMed Central. Gastrointestinal Adverse Effects of GLP-1 and Dual GLP-1/GIP Receptor Agonists: A Comprehensive Update in Diabetic and Obese Populations
20
PubMed Central. Early Weight Regain After GLP-1 Receptor Agonist Discontinuation: Mechanisms and Implications for Treatment De-Escalation Strategies
21
PubMed Central. A Comprehensive Review on Weight Gain following Discontinuation of Glucagon-Like Peptide-1 Receptor Agonists for Obesity
22
PubMed. Adaptations of glucagon-like peptide-1 receptor agonist-induced weight loss, drivers of weight regain and future directions of therapeutics
23
PubMed. Muscle Mass and Glucagon-Like Peptide-1 Receptor Agonists: Adaptive or Maladaptive Response to Weight Loss?
24
PubMed Central. Effects of Tirzepatide on Skeletal Muscle Mass in Adults: A Systematic Review
25
PubMed Central. Beyond Fat Loss: Addressing the Sarcopenia Challenge of Incretin-Based Therapies
26
Medicina Clínica (English Edition). GLP-1 receptor agonists and GIP/GLP-1 co-agonists in the treatment of obesity in adolescents and the elderly
27
PubMed Central. GLP-1 Agonists in Adolescent Obesity: A Narrative Review of Single, Dual, and Triple Agonists
28
PubMed. Novel dual GLP-1/GIP receptor agonists show neuroprotective effects in Alzheimer’s and Parkinson’s disease models
29
PubMed. A novel dual GLP-1 and GIP receptor agonist is neuroprotective in the MPTP mouse model of Parkinson’s disease by increasing expression of BNDF
30
PubMed. Neuroprotective effects of a triple GLP-1/GIP/glucagon receptor agonist in the APP/PS1 transgenic mouse model of Alzheimer’s disease
31
Elsevier / Peptides. Molecular evolution of GIP and Exendin and their receptors